Depression is common, particularly among the elderly. It sometimes heralds, and frequently accompanies, other diseases such as Alzheimer's or Parkinson's disease which are more prevalent in old age. Depression leads to significant morbidity and mortality, especially among the elderly, through difficulties with activities of daily living, worsening of medical problems, and an increased rate of suicide. However, study of the functional brain imaging correlates of idiopathic (or endogenous) depression is difficult, perhaps because of clinical and biological heterogeneity. Depression in neuropsychiatric disorders, because the neuropathology is well characterized, can serve as a model for idiopathic depression. We propose to study the phenomenology and functional brain imaging correlates of depression in two neuropsychiatric diseases with well-characterized clinical syndromes and neuropathology (Alzheimer's and Huntington's disease) using single photon emission tomography (SPECT). Such comparative studies will address the potential existence of diseasespecific neurophysiologic and symptom profiles in depression. Depression in Alzheimer's and Huntington's disease is associated with specific disturbances in cerebral blood flow as measured by single photon emission tomography. Specifically, depressed patients with AD and HD will have in common inferior frontal hypoperfusion compared to non-depressed patients with these diseases. We predict blood flow abnormalities will be characteristic of the symptomatic state. To determine the functional brain imaging (AD) and Huntington's (HD). We will compare depressed and non-depressed subjects with these diseases by measuring cerebral blood flow by single-photon emission tomography (SPECT), and evaluate patients on scales of depression, dementia, and functional disability.